The prevalence rates of asthma, IgE-mediated sensitization and allergic diseases vary throughout the world, and are highest in the English speaking nations and in the developed countries. There has been 2-3-fold increase in asthma prevalence in the latter part of the 20th century. More recent evidence suggests that the increase is asthma may have flattened off, and that in some age groups asthma rates may be decreasing in the areas of high prevalence. However, in many parts of the world asthma prevalence continues to increase, and the global differences may be getting smaller.
Epidemiology of Allergic Diseases. Available from: https://www.researchgate.net/publication/303414662_Epidemiology_of_Allergic_Diseases [accessed Nov 26 2017].